Curable silicone RTV compositions based upon hydrosilation cure of two part mixtures of silicones having ethylenic unsaturation (usually vinyl or allyl) and silicones having silicon hydride groups are well known. Cure of such compositions is catalyzed by platinum compounds. Such compositions cure rapidly at room temperature but provide very little adhesion to most substrates. Such compositions are widely used as release coatings, sealants, and as vibration dampening compounds.
One vibration dampening use of such compositions occurs in jet engines where the compositions are applied to parts which may see temperatures as high as 350.degree. F. in operation. In the course of manufacture of the engine, the composition will be exposed to elevated temperatures, such as a bake cycle, but the composition must be curable at room temperature in order to allow subsequent manufacturing operations to take place before baking. Thus, while it is not critical that the final properties be obtained at room temperature, the composition must cure within a reasonable time at room temperature in order to be suited for this application.
Substrates to which the hydrosilation-cured vibration dampening compound may be applied include: ferrous metals such as steel or stainless steel, aluminum and aluminum alloys, titanium and polymer/fiber composite materials such as Ultem.TM., a glass fiber reinforced polyimide sold by General Electric, Plastics Div. In this application it is important that the cured vibration dampening composition have good adhesion to the substrates to which it is bonded. Adhesion is particularly poor with aluminum alloys, titanium and composite materials, and consequently, it has in the past been the practice to carefully clean the substrates with a solvent such as methyl ethyl ketone, ethanol or isopropyl alcohol, and then to apply an adhesion improving primer, for instance a silane/titanate primer such as Dow Coming 1200 Primecoat or Loctite VSI V-06 primer, before applying the vibration dampening compound to the substrate. The use of a primer, however, increases the cost of materials and labor and lengthens manufacturing time. Further, worker exposure to primer solvent or to primer active ingredients is undesirable. Consequently, there is need to provide improved room temperature vulcanizing silicone compositions capable of withstanding temperatures of up to 350.degree. F. which provide better primerless adhesion to at least some of these substrates.
Solving this problem by addition of adhesion promoters to the composition is not a simple matter. For instance it is known to use silanes having hydrolyzable groups and a nonhydrolyzable polar functional group as adhesion promoters for many adhesive formulations. Examples of such materials include 3-methacryloxypropyltrimethoxysilane, vinyltriethoxysilane and 3-aminopropyltriethoxysilane. Some of these materials have been found to provide reasonable improvement in an adhesion to aluminum, aluminum alloys and titanium substrates, when incorporated into known vibration dampening hydrosilation cured RTV formulations but not on plastics. Other materials known to improve adhesion of RTV silicones to substrates, are well known inhibitors of hydrosilation at room temperature.
In U.S. Pat. Nos. 4,256,870; 4,340,647 and 4,476,166, it is disclosed that diallyl maleate is an effective inhibitor at room temperature of precious metal catalyzed hydrosilation cure reactions at levels of 0.1%-0.5% or greater.
In U.S. Pat. No. 3,882,083, it is disclosed that triallyl isocyanurate functions as an inhibiter of room temperature cure of platinum catalyzed hydrosilation cured silicone formulations at levels less than 0.3%.